Chapter 7: The Formation of Planetary Systems

Vocabulary terms and definitions covering the formation of the Solar System, planetary dynamics, and exoplanet detection methods based on Chapter 7 lecture materials.

Planetary system

A system comprised of a star, planets, moons, and smaller bodies.

Nebular hypothesis

The theory first proposed by Immanuel Kant and Pierre-Simon Laplace that a rotating cloud of interstellar gas collapsed and flattened to form a disk, with the Sun at the center and planets forming from the disk.

Self-gravity

The attraction between parts of an object that pulls outer layers toward the center.

Hydrostatic equilibrium

A state in which inward gravitational forces and outward pressure forces (like gas or radiation pressure) are balanced, ensuring stars and planets are stable.

Protoplanetary disk

A rapidly rotating, flattened structure of gas and dust (constituting roughly $1\%$ of the initial mass) that surrounds a young star and eventually forms planets.

Protostar

A large ball of gas that has not yet become hot enough to be a star, containing about $99\%$ of the mass of the original collapsing cloud.

Tangential speed ($v$)

The linear speed of an object moving along a circular path, expressed as $v=r\omega$.

Rotational speed ($\omega$)

Also called angular speed, it is the number of rotations or revolutions per unit of time, commonly measured in RPM.

Rotational inertia

Also known as the moment of inertia, it is the property of an object to resist changes in its rotational state of motion, depending on mass and its distribution about the axis.

Angular momentum ($L$)

A conserved quantity for rotating objects defined by the mass, velocity, and size of the system ($L = m \times v \times r$).

Planetesimals

Particles roughly $1\,\text{km}$ in size formed when small grains of gas and dust collide and stick together; these serve as the building blocks of planets.

Refractory materials

Substances such as rocks and metals that do not melt at high temperatures and are typically found in the inner disk.

Volatile materials

Materials like water ($H_2O$), ammonia ($NH_3$), and methane ($CH_4$) that can melt or evaporate at moderate temperatures and are found in the outer disk.

Organic materials

Substances containing carbon-hydrogen bonds, such as methane ($CH_4$) and carbon monoxide ($CO$).

Planet migration

The process by which a planet changes its orbit after formation, often due to a loss of angular momentum or gravitational encounters with other large bodies.

Primary atmosphere

The initial envelope of low-mass gases, primarily hydrogen and helium, gathered by a planet from the protoplanetary disk.

Secondary atmosphere

The atmosphere of a low-mass planet formed after its primary atmosphere is lost, created by volcanic outgassing of heavy gases and impacts from volatiles like comets.

Core accretion–gas capture

The model of giant planet formation where planetesimals grow large enough for their gravity to capture and hold nearby gas from the disk.

Disk instability model

An alternative 'top-down' model for planet formation where the protoplanetary disk fragments into massive clumps that collapse to form planetary cores.

Dwarf planets

Round objects that orbit the Sun but have not cleared their orbital paths of smaller bodies.

Asteroids

Small bodies found primarily inside the orbit of Jupiter.

Comet nuclei

Icy planetesimals that survived planetary accretion in the outer Solar System.

Exoplanet

A body with a mass less than $13$ Jupiter masses that orbits a star other than the Sun.

Brown dwarf

Often called a 'failed star,' an object with a mass between $13$ and $80$ Jupiter masses ($0.08$ solar masses).

Radial velocity method

An exoplanet detection technique that measures periodic Doppler shifts in a star's light caused by the gravitational pull of an orbiting planet.

Transit method

A method to find exoplanets by detecting the periodic decrease in a star's brightness as a planet passes in front of it; it allows for the calculation of the planet's radius.

Coronagraph

An occulting mask fitted inside a telescope to block the light of a star so that faint nearby planets can be seen.

Starshade

An external occulter flown in conjunction with a space telescope to block starlight and reveal orbiting exoplanets.

Microlensing

The process where a planet's gravity focuses light from a background star, making it temporarily brighter and allowing for the detection of distant or wide-orbit planets.

Astrometry

A detection method that precisely measures the change in a star's position in the sky to infer the presence of an orbiting planet.

Hot Jupiters

Jupiter-sized, volatile-rich exoplanets found in very close orbits to their host stars, likely due to planetary migration.

Mini-Neptunes

Gaseous exoplanets with masses between $2$ and $10$ times that of Earth.

Super-Earths

Rocky exoplanets that are more massive than Earth (greater than $1$ Earth mass).

Rogue planets

Free-floating planets that do not orbit a star, having been ejected from their original systems or formed independently in the interstellar medium.

Hycean world

A hypothetical type of exoplanet featuring a liquid water ocean beneath a hydrogen-rich atmosphere.

Habitable zone

The region around a star where the distance is exactly right for liquid water to potentially exist on a planet's surface.